Esters are compounds which currently find use in such areas as pesticides and herbicides, metal extraction agents, synthetic lubricants, polymerization aids for acrylic acid esters, insect attractants and repellants, industrial fragrances, odorants and cosmetic components, pharmaceutical applications, and photographic applications. Esters are typically made from a two step process. First, the corresponding carboxylic acid is produced. The acid is then reacted with an alcohol to produce the desired ester via a condensation process.
High volume production of dialkyl esters (two alkyl groups at the α-carbon) and trialkyl esters (three alkyl groups at the α-carbon) can be very difficult to prepare from the corresponding acid because of the hydrolytic instability of the product esters. Koch and Moller (U.S. Pat. No. 2,967,873) describe a synthesis of trialkyl esters from olefins using a catalyst system of BF3.H2O.ROH where ROH is an aliphatic alcohol of low molecular weight and an olefin having six or more carbons. However, this process always produces some carboxylic acid along with the desired ester and requires continual adjustment of the water to alcohol ratio in the recycled BF3 catalyst.
Commercial use of this technology is currently employed by Exxon Chemical Company (Baton Rouge, La.) and Shell (Pernes, Holland). ExxonMobil's products are known as “neo acids” while Shell's products are called Versatic™ acids. ExxonMobil Chemical employs BF3.2H2O as catalyst and Shell employs H3PO4.BF3.H2O in a 1:1:1 ratio (J. Falbe, “New Synthesis with Carbon Monoxide”, Springer-Verlag, 1980, p. 406). Olefins used in these processes include isobutylene, propylene oligomers and C8-C11 fractions. The major commercial products are 1,1,1-trimethyl acetic acid (pivalic acid or neopentanoic acid) and neodecanoic acid or Versatic™10. The major disadvantage of these processes is the difficulty, relatively high cost, and process inefficiencies in recycling the acid catalyst in the process.
Gelbein (Re. 31,010) discloses a one step process for the preparation of esters from olefins in the presence of BF3.CH3OH. This process requires that uncomplexed BF3 be distilled from the reaction products. This distillation is very inefficient and requires the use of corrosion resistant processing equipment. Following the distillation of uncomplexed BF3, methanol is added to form an azeotrope with the desired ester and a BF3.2CH3OH adduct. The distilled BF3 or fresh BF3 is added to BF3.2CH3OH to form BF3.CH3OH, which is recycled to the reaction unit. The BF3.CH3OH is a preferred because it is stronger acid than BF3.2CH3OH, and the esterfication of propylene or ethylene can occur at temperatures below 100° C., preferably below 60° C.
Jung and Peress (U.S. Pat. No. 4,311,851) also disclose the preparation of esters from a BF3.ROH complex catalyst. This process also requires that uncomplexed BF3 be distilled from the reaction products and then recycled to form the active BF3.ROH catalyst.
Large volume, commercial scale production of dialkyl and trialkyl esters remains a problem in the chemical industry. Presently, the production of these esters, particularly trialkyl esters (neo acid esters), are limited by having to use a relatively large amount of purified acid catalyst and/or by having to distill off a corrosive strong acid (HF or BF3) from the reaction products so the acid can be recycled in the process. Distillation of this acid requires special operational handling and specialized process units which results in a highly inefficient process.